1. Technical Field
The present invention generally relates to a display device having an input sensing function such as a touch-panel-type liquid crystal device. In particular, the invention relates to a liquid crystal device that has a touch panel function that allows a user to input a variety of kinds of information by using a pointing means as an inputting object. Specifically, a liquid crystal device that has a touch panel function to which the invention relates is capable of detecting a pointing means such as a finger of a user, though not limited thereto, which points to an image display surface thereof on which an image is displayed, thereby allowing the user to input a variety of kinds of information by using the pointing means as an inputting object. In addition, the invention further relates to an electronic apparatus that is provided with such a liquid crystal device. Moreover, the invention further relates to an image sensor that realizes such detection.
2. Related Art
In the technical field to which the present invention pertains, a variety of display devices having a so-called touch panel input function has been proposed so far. In the configuration of a touch-panel-type liquid crystal device, which is an example of such a display device having a touch panel input function, an optical sensor (i.e., light sensor, photo sensor) is provided for either each of a plurality of pixel units or each of a plurality of groups of pixel units, where each group thereof is made up of a given number of pixel units. With such a configuration, in addition to its basic function of displaying an image by using light that transmits through the pixel units, a liquid crystal device having a touch panel function of the related art allows a user to input information by means of a pointing means such as a finger of a user, though not limited thereto (In the following description, the pointing means may be referred to as a “pointing object” with no intention to limit the technical scope of the invention as long as the context allows). The liquid crystal device having a touch panel function of the related art allows a user to input information through the functioning of photo detectors, which are light-sensitive pickup elements. Specifically, the photo detectors such as optical sensors detect either the touching of a variety of pointing objects such as a finger of a user or other pointing member, though not limited thereto, onto the display surface of the liquid crystal device or the moving of such a pointing object over the display surface of the liquid crystal device. By this means, the user can input information into the liquid crystal device. For example, a non-patent document of “Touch Panel Function Integrated LCD Using LTPS Technology”, N. Nakamura et al., IDW/AD'05 p. 1003-1006 discloses a liquid crystal device that is capable of displaying images under the operation of a driving circuit that is made up of thin film transistors (TFT) containing low-temperature polysilicon (LTPS); the liquid crystal device that is described in the above-identified non-patent document has a touch panel function that enables various kinds of information to be inputted therein on the basis of the image of a pointing object that is detected by an optical sensor provided in each pixel.
In the circuit configuration of an optical sensor that is mounted on such a touch-panel-type liquid crystal device of the related art, for example, a photodiode and a capacitor (i.e., capacitance) are electrically connected to each other. Electric charge that has been accumulated in the capacitor is discharged in response to a photocurrent that is generated in a photodiode that has received incident light. Then, on the basis of electric potential (i.e., voltage) that changes due to the discharging of electric charge, the gradation level of an image is identified. Specifically, for example, one group of optical sensors that is arrayed in a “pointing-means-overlapping” region of an image display area in which images are displayed (i.e., one group of optical sensors that is provided in a partial region thereof at which the pointing means overlap), which can be paraphrased as one group of optical sensors that is provided in a partial region thereof on which the shade of the pointing means is projected, detects the amount of incident light that corresponds to the shade of the pointing means. On the other hand, the other group of optical sensors that is arrayed in the remaining region thereof at which the pointing means does not overlap detects the amount of incident light while taking external light that is not shut off by the pointing means as incident light. Since the amount of the incident light that is detected by the former group of optical sensors, which is arrayed in the partial region of the image display area at which the pointing means overlap, differs from the amount of the incident light that is detected by the latter group of optical sensors, which is arrayed in the remaining region of the image display area at which the pointing means does not overlap, a touch-panel-type liquid crystal device of the related art acquires an image having a difference in its gradation level for each image region on the basis of the difference in the amount of light therebetween. Therefore, in the configuration of such a liquid crystal device having a touch panel function of the related art, a plurality of the optical sensor detects the amount of incident light that enters through the display surface where images are displayed. Then, the touch-panel-type liquid crystal device of the related art acquires an image having image portions (i.e., regions) whose gradation levels are determined on the basis of the amount of incident light that is detected by each of the plurality of the optical sensors, thereby making it possible to identify the location of a pointing means.
The effective detection range of the optical sensor that is mounted in the touch-panel-type liquid crystal device of the related art within which the optical sensor can detect the amount of incident light, or in other words, a range of the amount of incident light within which it can generate a photocurrent that is in accordance with the amount of the incident light is specified as the design value thereof. Therefore, if the optical sensor receives incident light that has a greater light-amount value that goes beyond the effective detection range thereof, a photocurrent that is generated in accordance with the amount of incident light saturates. As a result of such saturation, a voltage change that is supposed to occur on the basis of the photocurrent does not actually occur. Therefore, in such a case, it is not possible to discriminate the target-image region of a pointing means from other non-target-image region.
In addition, if other object, which is not the pointing means, overlaps the pointing means over the display surface of the touch-panel-type liquid crystal device of the related art, it is not possible to discriminate the shade of the pointing object and the shade of the other portion from each other.
If the target image is formed only in either one of a white image region (i.e., a bright image portion having a high gradation level) and a black image region (i.e., a dark image portion having a low gradation level) in accordance with the amount of incident light that is received by each of the plurality of optical sensors, which is a hypothetical case, it is conceivable to adjust the gradation levels of the acquired image in such a manner that the target-image region of the pointing means can be discriminated from the other non-target-image region, which is achieved by uniformly adjusting the amount of incident light that enters each of the plurality of optical sensors arrayed in the display area so as to ensure that the amount of incident light detected by the optical sensors falls within the effective detection range.
The touch-panel-type liquid crystal device of the related art has, as its typical constituent elements, two polarizing plates that sandwich liquid crystal and a pair of substrates thereof. JP-A-2003-330013 discloses a liquid crystal device that is provided with substrates having built-in polarizing plates.
The touch-panel-type liquid crystal device of the related art has not provided an effective technical solution to the problem of difficulty in the identification of the location of a pointing means under unfavorable ambient conditions. Specifically, in a case where an image containing a white image portion and a black image portion is acquired due to reasons attributable to the ambient conditions of a pointing means such as the optical intensity of external light or the presence of other portion that overlaps the pointing means, that is, a noise, it is difficult to make a predetermination as to whether the target-image region corresponding to the pointing means is contained in the white image portion or the black image portion. In such a case, it is difficult to identify the location of the pointing means.
If the image data of an acquired image that contains the target-image portion of a pointing means has a gradation level that is sufficient for detecting the target-image portion of the pointing means, it is possible to discriminate the target-image region of the pointing means from other non-target-image region by performing various kinds of arithmetic processing on such image data. However, if incident light detected by the optical sensor has a light-amount value that does not fall within the effective detection range of the optical sensor, it is not possible to even acquire image data itself containing gradation data that allows the target-image portion of the pointing means to be identified through computation processing.
Similar to the typical configuration of an image pickup apparatus such as a camera that has a mechanical incident-light amount/intensity/level reduction structure (e.g., mechanical iris) and a shutter mechanism that is provided in the optical propagation path thereof, it is conceivable to provide an iris mechanism and a shutter mechanism in each of the optical sensors of the touch-panel-type liquid crystal device of the related art. However, it is practically impossible or at best difficult to secure a space for providing such an iris mechanism and a shutter mechanism at the light-reception side in each of the optical sensors along the optical propagation path of incident light. Among other reasons, in the configuration of the touch-panel-type liquid crystal device of the related art, because it is necessary to provide optical sensors in the image display area thereof, it is practically impossible or at best difficult to secure a space for providing such an iris mechanism and a shutter mechanism at the light-reception side in each of the optical sensors along the optical propagation path of incident light without sacrificing the display performance thereof. Specifically, it is practically impossible or at best difficult to secure such a space without narrowing open regions through which display light that actually contributes to image display can be transmitted.
In addition, similar to the above-described technical problem of a liquid crystal device having a touch panel function of the related art, an image sensor of the related art that picks up (i.e., acquires, captures, or detects) an image of a detection target object has not yet addressed the technical problem of difficulty in discriminating the image region of the detection target object from other region.